1. Field of the Invention
The present invention relates to an atomic oscillator, and particularly to an atomic oscillator whose resonance frequency derives from energy transitions of rubidium atoms.
2. Description of the Related Art
Rubidium atomic oscillators provide a constant frequency output by taking advantage of a highly stable resonance frequency of rubidium (Rb) atoms. Because of their extremely high frequency stability, rubidium oscillators are widely used as a frequency standard for television broadcast services and also as a high-accuracy timing source for digital synchronous networks and mobile communications systems. A high degree of accuracy is not the only thing that the customers demand; the market always seeks more compact and less costly solutions for rubidium frequency standards.
FIG. 18 shows a typical basic structure of an existing rubidium atomic oscillator. The illustrated rubidium atomic oscillator 100 is formed from a voltage-controlled crystal oscillator (VCXO) 101, a radio-frequency (RF) signal synthesizer 102, an atomic resonator 103, and a frequency controller 104.
The VCXO 101 is an electrically tunable oscillator, whose output frequency is determined by an external control voltage provided from the frequency controller 104. Besides being available for external use, the oscillation signal is supplied to the frequency synthesizer 102 for control purposes. In the frequency synthesizer 102, the given VCXO signal is phase-modulated with a low-frequency signal supplied from the frequency controller 104. The modulated signal then undergoes a process of frequency synthesis, which produces an RF signal of 6.83469 . . . GHz, the resonance frequency (natural frequency) of rubidium.
The atomic resonator 103 outputs a resonance detection signal as a response to the RF signal supplied from the frequency synthesizer 102. The frequency controller 104 has an internal low-frequency oscillator to create a low-frequency signal for modulation of the VCXO signal in the RF signal synthesizer 102. The same low-frequency signal is used to demodulate the resonance detection signal, which is the technique known as synchronous detection. The resulting control voltage is used to stabilize the frequency of VCXO 101.
As can be seen from the above, the rubidium atomic oscillator 100 regulates the VCXO output frequency, based on a resonance detection signal of the atomic resonator 103. It can therefore produce an oscillation signal that is as steady as the resonance frequency of rubidium atoms.
Since the oscillator output is supposed to serve as a reference clock signal for external circuits, it would be preferable if the oscillator can provide a particular frequency that is required. Actually, rubidium's resonance frequency is 6.83469 . . . GHz, which is not a simple number, but has many trailing digits. Consider, for example, that an external circuit needs a clock signal of 10 MHz. To meet the requirement, an appropriate integer multiple of 10 MHz is chosen as the oscillation frequency of the VCXO 101, within a frequency range where the VCXO 101 can work most stably. The VCXO output also serves as the seed frequency from which a frequency synthesizer produces an atomic resonance frequency signal of 6.83469 . . . GHz. Typically, a direct digital synthesizer (DDS) is employed to create an appropriate frequency from the integer multiple of 10 MHz. DDS devices offer flexible frequency setting capabilities, and are available in a single chip version. The created signal is then modulated and upconverted by a phase-locked loop (PLL) to yield an RF signal for driving an atomic resonator 103.
FIG. 19 is a block diagram of a rubidium atomic oscillator with a DDS-based RF signal generator. This rubidium atomic oscillator 200 comprises a VCXO 201, an RF signal synthesizer 202, an atomic resonator 203, a frequency controller 204, and a frequency divider 205. The RF signal synthesizer 202 is composed of a DDS 202a, a modulator 202b, a PLL 202c, and a frequency multiplier 202d. The example of FIG. 19 assumes the output frequency of 10 MHz for external use. The VCXO 201 is thus designed to produce a 20 MHz oscillation signal for use as a clock signal of the DDS 202a. Based on this 20-MHz signal, the DDS 202a synthesizes a lower-frequency signal of 4.952 . . . MHz, which is the result of integer division of rubidium atomic resonance frequency (i.e., 6.83469 . . . GHz/1380). The modulator 202b then modulates this signal and the PLL 202c and frequency multiplier 202d upconvert the modulated signal to yield an RF signal of 6.83469 . . . GHz. To be more specific, the PLL 202c contains a voltage-controlled oscillator (VCO) that produces a signal of 2.278 . . . GHz, and the modulated signal of 4.952 . . . MHz is used as a reference signal for the VCO. The frequency multiplier 202d triples this frequency, thus outputting 6.83469 . . . GHz. The frequency divider 205, on the other hand, halves the VCXO frequency, thus producing a 10-MHz signal for external use. Other circuit blocks shown in FIG. 19 operate in the same way as we described earlier in FIG. 18.
One example of an oscillator using frequency synthesizers is shown in the Japanese unexamined patent publication No. 3-235422 (1991), pages 1 to 3, FIG. 1. The oscillator generates an RF signal by mixing the outputs of a frequency synthesizer and a frequency multiplier, both of which operate with a source signal from a crystal oscillator. Instead of changing the division ratio of a single frequency synthesizer, the proposed oscillator employs a plurality of frequency synthesizers with different division ratios, so that one of their outputs will be subjected to the subsequent frequency mixing operation. The proposed oscillator design eliminates undesired transient response that a frequency synthesizer would make when it attempts to resynchronize itself in order to operate with a new frequency division ratio. The use of multiple synthesizers, however, increases the size of oscillator circuits.
Referring again to the rubidium atomic oscillator 200 of FIG. 19, the VCXO 201 supplies its oscillation signal to the DDS 202a as a clock input signal, and the output of this DDS 202a serves as a reference source for the PLL 202c. The frequency multiplier 202d multiplies the upconverted signal, thereby producing an RF signal. This conventional circuit arrangement is not optimal in terms of noise and spurious components that could be contained in the RF signal, as will be discussed below.
However successful the circuit design is in stabilizing the frequency, the oscillation signal of the VCXO 201 contains a certain amount of jitter (also referred to as “phase noise”). The problem with the conventional oscillator 200 is that the VCXO jitter is multiplied by the frequency multiplier 202d, together with the oscillation signal, resulting in a larger amount of jitter observed at the RF signal output. In addition, spurious components inherent in the DDS output could be another source of noise. For those reasons, the RF signal produced in the conventional oscillator 200 is contaminated with a considerable amount of noise, which causes degradation of signal-to-noise (S/N) ratio in detecting resonance of the atomic resonator 203. This problem holds also for the oscillator disclosed in the Japanese unexamined patent publication No. 3-235422 mentioned above. That is, the oscillator contains a multiplier to upconvert a crystal oscillator output. This means that the mixer receives an oscillation signal with increased jitter, which results in a large phase noise.
The output frequency of a conventional rubidium atomic oscillator (e.g., the oscillator 200) can be made variable to allow fine tuning. The common method is to shift the atomic resonance frequency by manipulating a magnetic field (known as “C field”) in the atomic resonator 203. The problem here is that the curve of output frequency versus C-field strength is not linear. This non-linearity makes it difficult for users to control the output frequency of an atomic oscillator.
Another issue to consider is a drift of output frequency. Frequency drift of atomic oscillators stems from variations in the amount of rubidium lamp light due to temperature changes and aging of components used. For better long-term stability, the frequency conversion parameters of a DDS 202a has to be manipulated to compensate for the temperature variations or age deterioration. Conventional oscillators, including those shown in FIGS. 18 and 19 and Japanese unexamined patent publication No. 3-235422, lack the function of controlling such parameters, thus failing to avoid degradation of frequency stability.